Implicit Cursors in PL/SQL

PL/SQL is Oracle’s procedural extension to SQL and makes it efficient to develop database applications. It allows the managing of a query database using cursors, which are point

ers to the result sets of queries. Cursors in PL/SQL can be broadly classified into two categories: explicit cursors and implicit cursors. This article will focus on implicit cursors, Implicit Cursor Attributes in PL/SQL and its use is detailed along with its attributes. We will take a few examples to get a clearer understanding of how implicit cursors might optimize interactions with the database in PL/SQL.

Understanding of Implicit Cursors in PL/SQL

A cursor is a pointer in PL/SQL that can interface with the result set returned by a SQL query. An implicit cursor is automatically opened by PL/SQL any time a SQL statement such as SELECT, INSERT, UPDATE, or DELETE is executed. Unlike an explicit cursor, where the developer declares, opens, fetches, and closes the cursor, an implicit cursor handles all of these operations automatically. This makes coding much simpler, particularly with single-row operations or simple DML statements.

What Are Implicit Cursors?

An implicit cursor is also automatically established and maintained by PL/SQL whenever a SQL statement is executed. The cursor will point to the memory location where the result set of the SQL query is stored. For example, the insertion operation is managed in the background by an implicit cursor that gets automatically established whenever you execute an INSERT statement.

Key features of implicit cursors include:

• Automatic management: Developers do not need to declare or explicitly manage implicit cursors.
• Efficient for simple queries: Implicit cursors are ideal for single-row queries or simple DML statements.
• Use of attributes: Implicit cursors have attributes like %FOUND, %NOTFOUND, %ROWCOUNT, and %ISOPEN that allow developers to monitor the outcome of the SQL operation.

How Implicit Cursors in PL/SQL Work

What actually happens is the implicit cursors invoked while any DML statement is issued. It undergoes three steps:

• Execution of SQL statement: With the execution of a SQL query, like SELECT, INSERT, UPDATE, and DELETE any time, an automatic implicit cursor is created by PL/SQL for taking care of that statement.
• Results Fetching: The cursor implicitly fetches the results of queries that return data, such as SELECT, into memory.
• Closing the cursor: The implicit cursor is automatically closed when the SQL operation is complete. The developer does not have to open and close it himself.

Implicit Cursor Flow

The following flow summarizes how an implicit cursor operates in PL/SQL:

1. SQL statement execution
2. Creation of an implicit cursor
3. Fetching of the result set (if applicable)
4. Execution of DML operations
5. Cursor automatically closed after the operation is complete

Implicit Cursor Attributes in PL/SQL

Implicit cursor attributes in PL/SQL are essential for understanding the results of SQL statements executed without explicitly defined cursors. These attributes provide valuable information regarding the status of DML operations, such as INSERT, UPDATE, and DELETE. For instance, the %FOUND attribute indicates whether the last operation affected any rows, while %NOTFOUND serves as the logical opposite, signaling if no rows were impacted. Additionally, the %ROWCOUNT attribute reveals the number of rows affected by the most recent SQL statement. By leveraging implicit cursor attributes in PL/SQL, developers can efficiently track the outcomes of their data manipulation operations and implement appropriate error handling or conditional logic based on these results. Understanding these implicit cursor attributes in PL/SQL is crucial for writing robust and effective database applications that respond accurately to various data conditions.

• %FOUND: Returns TRUE if a SQL statement changes the contents of one or more rows.
• %NOTFOUND: Returns TRUE if a SQL statement affects no rows.
• %ROWCOUNT: Returns the number of rows affected by an SQL statement.
• %ISOPEN: Returns TRUE if the cursor is still open (although implicit cursors are automatically closed after execution, so this typically returns FALSE).

These attributes can be used to control the flow of your PL/SQL program based on the outcome of the SQL operation.

Table 1: Implicit Cursor Attributes

Attribute	Description
%FOUND	Returns TRUE if a DML statement affects one or more rows.
%NOTFOUND	Returns TRUE if a DML statement affects no rows.
%ROWCOUNT	Returns the number of rows affected by the last SQL operation.
%ISOPEN	Returns TRUE if the cursor is open (always FALSE for implicit cursors).

Examples of Using Implicit Cursors in PL/SQL

Implicit cursors are automatically used whenever a SQL statement is executed in PL/SQL. Let’s explore some examples to better understand how they work in practice.

Example 1: Basic Use of an Implicit Cursor

In this example, we perform a simple INSERT operation using an implicit cursor.

DECLARE
    v_emp_id employees.employee_id%TYPE := 120; 
    v_first_name employees.first_name%TYPE := 'John';
    v_last_name employees.last_name%TYPE := 'Doe';
BEGIN
    -- Insert a new employee record into the employees table
    INSERT INTO employees (employee_id, first_name, last_name)
    VALUES (v_emp_id, v_first_name, v_last_name);
    
    -- Check if the insertion was successful
    IF SQL%FOUND THEN
        DBMS_OUTPUT.PUT_LINE('Employee inserted successfully.');
    ELSE
        DBMS_OUTPUT.PUT_LINE('No row inserted.');
    END IF;
END;

In this example:

• We insert a new employee record into the employees table.
• After the insertion, the %FOUND attribute is used to check whether the operation affected any rows. If the insertion is successful, it prints a success message.

Example 2: Using Implicit Cursor Attributes

Here, we demonstrate how to use implicit cursor attributes to check the result of a DELETE operation.

DECLARE
    v_emp_id employees.employee_id%TYPE := 120; 
BEGIN
    -- Delete the employee record with employee_id = 120
    DELETE FROM employees WHERE employee_id = v_emp_id;
    
    -- Check if the deletion was successful
    IF SQL%FOUND THEN
        DBMS_OUTPUT.PUT_LINE('Employee deleted successfully.');
    ELSE
        DBMS_OUTPUT.PUT_LINE('No employee found with that ID.');
    END IF;

    -- Display the number of rows affected
    DBMS_OUTPUT.PUT_LINE('Number of rows deleted: ' || SQL%ROWCOUNT);
END;

In this example:

• We delete an employee from the employees table using their employee_id.
• The %FOUND attribute checks whether the delete operation affected any rows.
• The %ROWCOUNT attribute displays how many rows were deleted (which will either be 1 or 0 in this case).

Implicit Cursor Attributes in PL/SQL

Implicit cursor attributes are very important when you want to determine the outcome of a SQL statement. Now let’s take a little closer glance at these attributes in PL/SQL to understand how important they are in programming.

1. %FOUND
The %FOUND attribute returns TRUE if the last DML statement-executed INSERT, UPDATE or DELETE-affect rows for one or more rows. This attribute is useful if you want to make sure that an operation was performed.

2. %NOTFOUND
The %NOTFOUND attribute is the converse of %FOUND. It returns TRUE if no rows were affected by an SQL operation. It is often used in loops, or conditional checks to determine whether some action should be performed next.

3. %ROWCOUNT
The %ROWCOUNT attribute returns the number of rows affected by the SQL statement. It’s commonly used in all operations where there might be more than one row updated, deleted, or selected, giving an easy method to test for the scope of the operation.

4. %ISOPEN
The %ISOPEN attribute indicates if the cursor is open. For implicit cursors, this attribute always returns FALSE because implicit cursors are automatically closed when the SQL operation completes.

Comparison of Implicit and Explicit Cursors

Because implicit cursors take care of everything for you, you can rely on PL/SQL to work it out. In explicit cursors, you declare it, open it, fetch from it, and close it. Let’s compare them to know how to use each one properly.

Implicit vs Explicit Cursors Table 2

Feature	Implicit Cursor	Explicit Cursor
Declaration	Automatically created by PL/SQL	Manually declared by the developer
Management	Fully managed by PL/SQL (automatic)	Requires manual open, fetch, and close
Control	Less control over the cursor’s behavior	Full control over the cursor’s lifecycle
Complexity	Simpler to use for straightforward queries	More complex, but provides flexibility for multi-row queries
Usage	Best for single-row or simple DML operations	Ideal for handling multiple-row result sets

Implicit cursors are ideal for simple queries where minimal control is needed. However, for complex queries involving multiple rows, explicit cursors offer greater control and flexibility.

Common Errors with Implicit Cursors

Implicit cursors are easy to use, but certain errors can occur if not used correctly. Below are some common mistakes and how to avoid them:

1. Assuming Multiple Rows: Implicit cursors are designed for single-row operations. If you execute a SELECT query that returns multiple rows, an error will occur unless you handle the result set appropriately.
2. Forgetting to Use Cursor Attributes: Failing to check cursor attributes like %FOUND or %NOTFOUND can lead to incorrect logic flow, especially when the SQL operation affects no rows.
3. Not Understanding %ROWCOUNT Behavior: %ROWCOUNT returns the number of rows affected by the SQL operation. In some cases, developers mistakenly assume it always returns 1, leading to incorrect assumptions in logic.

Optimizing Performance with Implicit Cursors

To optimize performance when using implicit cursors, consider the following strategies:

• Minimize SQL Overhead: Implicit cursors are automatically created and closed, reducing the overhead of manual cursor management. This can improve performance for small, frequently executed SQL statements.
• Use Batching: When performing repetitive DML operations, consider using BULK COLLECT or FORALL statements to reduce the number of context switches between the PL/SQL engine and the SQL engine, optimizing performance.
• Monitor %ROWCOUNT for Bulk Operations: When performing batch operations like UPDATE or DELETE, use %ROWCOUNT to ensure that the expected number of rows are affected. This can help identify inefficiencies in query performance.

Advantages of Implicit Cursors in PL/SQL

Implicit cursors in PL/SQL are automatically created by Oracle for SQL statements that return a single row or execute non-query operations such as INSERT, UPDATE, or DELETE. These cursors simplify database interaction by managing the process of fetching data, handling exceptions, and closing the cursor automatically. Below are the key advantages of using implicit cursors in PL/SQL:

1. Simplified Syntax

Implicit cursors require no explicit declaration or management, making them easy to use for simple queries. Developers don’t need to write additional code to handle opening, fetching, or closing the cursor, as Oracle handles this automatically.

2. Reduced Development Time

Since implicit cursors are managed automatically by PL/SQL, developers save time by not needing to handle cursor operations manually. This is especially beneficial for executing straightforward SQL queries where explicit cursors would add unnecessary complexity.

3. Automatic Handling of Single-Row Queries

Implicit cursors are ideal for SQL statements that return only one row, such as SELECT INTO. Oracle automatically fetches the result and assigns it to the variables, ensuring that data is efficiently retrieved with minimal code.

4. Automatic Memory Management

Implicit cursors handle memory allocation and deallocation behind the scenes, reducing the risk of memory leaks or improper cursor closure. This helps improve the reliability of the PL/SQL code and reduces the chances of resource exhaustion.

5. Error Handling Integration

PL/SQL integrates implicit cursors seamlessly with its exception-handling mechanisms. If an error occurs during the execution of an implicit cursor, such as a NO_DATA_FOUND or TOO_MANY_ROWS exception, it can be handled easily within the exception block.

6. Improved Readability for Simple Operations

For straightforward queries or data manipulation operations, implicit cursors enhance code readability by removing the need for explicit cursor declaration and management. This makes the code cleaner and easier to understand for simple SQL operations.

7. Less Overhead for Simple DML Statements

Implicit cursors are efficient for executing INSERT, UPDATE, and DELETE statements, as the cursor management is done automatically by Oracle. This reduces the overhead of explicitly managing cursors, making the code more efficient for simple data manipulation tasks.

Disadvantages of Implicit Cursors in PL/SQL

While implicit cursors in PL/SQL offer simplicity and automatic management for single-row queries and basic data manipulation, they come with certain limitations. These disadvantages can impact performance, flexibility, and error handling in more complex scenarios. Below are the key disadvantages of using implicit cursors in PL/SQL:

1. Limited Control

Implicit cursors are automatically managed by Oracle, meaning developers have less control over cursor behavior. This lack of flexibility can be a drawback when more fine-tuned control is needed, such as with multi-row queries or custom fetch strategies.

2. Not Suitable for Multi-Row Queries

Implicit cursors are designed for operations that return a single row or for non-query statements. If a query returns multiple rows, using implicit cursors can lead to errors, such as the TOO_MANY_ROWS exception, as they cannot handle fetching multiple rows.

3. No Cursor Reusability

Implicit cursors are created and destroyed automatically for each SQL statement, which means they cannot be reused for multiple operations. This can result in inefficiencies, especially in scenarios where explicit cursors could be reused across several operations or iterations.

4. Limited Exception Handling for Complex Queries

While implicit cursors handle exceptions like NO_DATA_FOUND and TOO_MANY_ROWS, they may not provide sufficient detail or control for more complex error handling. In such cases, explicit cursors allow developers to implement more robust exception-handling mechanisms for better control over query execution.

5. Performance Overhead for Large Datasets

When dealing with large datasets, implicit cursors may introduce performance overhead since they are designed for single-row queries or simple DML operations. For bulk operations or queries that return multiple rows, explicit cursors or bulk processing mechanisms like FORALL and BULK COLLECT are more efficient.

6. Lack of Fetching Flexibility

Implicit cursors automatically fetch data in a single operation, which limits the ability to control how rows are fetched or processed. For example, explicit cursors allow developers to fetch rows one at a time or in batches, providing more flexibility in handling data retrieval.

7. No Cursor Attributes for Monitoring

Implicit cursors do not provide access to cursor attributes like %ROWCOUNT, %FOUND, %NOTFOUND, or %ISOPEN, which are available with explicit cursors. These attributes are useful for monitoring cursor operations and obtaining additional information about query results.

8. Not Ideal for Complex Logic

For queries involving complex logic or multiple fetch operations, implicit cursors may not be the best choice. Explicit cursors provide more flexibility for controlling the flow of data retrieval and handling more sophisticated scenarios that require custom logic or multi-row processing.